In general image sensors of the related art, electric charge accumulated in light receiving portions (or photodiodes) is read as signal voltages, and is subjected to A/D conversion (see, for example, Japanese Unexamined Patent Application Publication No. 2011-41091 (hereinafter “PTL 1”).
In an A/D conversion method described in PTL 1, in order to increase gradation precision and reduce conversion time, two A/D conversion circuits are connected to the same pixel output signal, and two reference voltages Vref1 and Vref2 having different slopes are individually input to the A/D conversion circuits from a reference voltage generation unit to execute A/D conversion at two different levels of gradation precision. In this method, however, the circuit area and power consumption may double. Thus, in another method described in PTL 1, one A/D conversion circuit is used and a determination unit is further provided. The determination unit determines the magnitude of the pixel output signal, and selects one of the two reference voltages having different slopes, namely, reference voltages Vref1 and Vref2, in accordance with the determined magnitude of the pixel output signal to apply different levels of conversion precision in accordance with the magnitude of the pixel output signal.